IMSDroid is the first fully featured open source SIP/IMS client for Android. It is also the only free Android SIP client which supports direct Video Calls.
3GPP IMS (IP Multimedia Subsystem) is the next generation network for delivering IP multimedia services. IMS is standardized by the 3rd Generation Partnership Project (3GPP). IMS services could be used over any type of network, such as 3GPP LTE, GPRS, Wireless LAN, CDMA2000 or fixed line.
The current version of IMSDroid partially implements GSMA Rich Communication Suite release 3 and The One Voice profile V1.0.0 (LTE/4G, also known as GSMA VoLTE) specifications.

· First version fully implementing RTCWeb standards (ICE, SRTP/SRTCP, RTCP-MUX...). This version can be used to call sipML5 webApp. To enable the features: Options -> General -> Media Profile -> RTCWeb

Linphone VideoLinphone is an open source internet SIP phone or Voice Over IP phone (VoIP) WIFI/3G, with video calls support for ARMv7 phones. Main features: -Audio with the following codecs: speex (narrow band and wideband), iLBC, G711, GSM -Video with mpeg4 and H264 codecs (ARMv7 only)

Skype (Android)With the Skype app, you can make free Skype-to-Skype calls, and send and receive IMs for free, one-to-one or with a group